This invention relates to heat engines and more particularly concerns a thermal energy converter that utilizes a material that changes configuration with temperature to convert thermal energy into a useful mechanical output.
A class of materials which may be termed Joule effect or shape memory materials exhibits a non-linear relation between stress and strain, which relation can be reversed by a temperature change, usually exhibiting an hysteresis effect. Thus, as first observed by the British physicist, James Joule, if a rubber material is first stretched at a relatively lower temperature and then warmed, it tends to contract. Many other materials, such as alloys of copper and zinc, aluminum or tin, indium and thallium, gold and cadmium, iron and platinum and some ternary alloys, such as copper, aluminum, and nickel also exhibit the Joule effect. A material that presently appears to be a most efficient Joule effect or shape memory material is known as Nitinol, a titanium nickel alloy having a 53.5-56.5% nickel content by weight. Such a material and its use in conversion of heat energy to mechanical energy is described in the U.S. patent to Buehler et al, U.S. Pat. No. 3,403,238.
Operation of several types of shape memory materials is described in an article on pages 934, 935 and 936 of Science Magazine, Volume 191, dated Mar. 5, 1976. Basically, these materials are formed into a desired configuration and annealed at a high temperature to "fix" the material shape. The material is then cooled, and at low temperature is deformed from this "fixed" or low strain configuration. Then, if the material is heated it will return to its "fixed" or low strain configuration. The force generated in the return or reshaping to its low strain configuration at a higher temperature is considerably greater than the force required to deform the material from its low strain configuration at a lower temperature.
A number of different types of heat engines employing shape memory material and in particular, Nitinol are described in a paper entitled "Nitinol Heat Engines" presented by A. D. Johnson to the Intersociety Energy Conversion Conference in August of 1975. This paper is found on pages 530, 531, 532, 533 and 534 of the IECEC 1975 record.
A number of Joule effect engines employing rubber tubing are described in an article in Scientific American of August 1970, pages 118, 119, 120, 121 and 122.
The prior Joule effect or shape memory heat engines are of limited output power or of a complexity that greatly limits practical application, or both. Such machines frequently involve arrangements such as shown in the U.S. patent to Lee, U.S. Pat. No. 3,303,642. The Lee patent describes a thermal engine which utilizes a conventional thermally responsive material rather than a Joule effect material and thus is capable of producing only limited amounts of output energy. Nevertheless, the arrangement of Lee illustrates the fact that the pulley type of thermal engine requires excessively large pulley diameters for increased powers. Further, such a device, like the eccentrically mounted wheels shown in the Scientific American article, are subject to high bearing loads which increase to very large values for devices providing larger amounts of output energy.
Accordingly, it is an object of the present invention to provide a heat engine of greater simplicity and lower cost which eliminates or minimizes problems of prior heat engines.